This invention relates to removal of excess magnetic developing material from a developed, magnetially latently imaged member.
Latently imaged members such as, for example, electrostatically latently imaged xerographic photoconductive members and latently megnatically imaged magnetographic imaging members are typically developed by deposition of developer material on the imaging member. In magnetic imaging the developing material is magnetic and attracted by magnetic fields to the latent magnetic image created in a magnetizable imaging member such as, for example, ordinary magnetic recording tape. In electrostatographic imaging systems such as, for example, xerography, the developing material typically comprises the two components of carrier and toner. The toner material is typically capable of becoming triboelectrically charged and, owing to this charge, is attracted to the charge pattern residing on the photoconductive imaging member. In either case, as a practical matter, developer material is attracted to and deposited upon the imaging member not only in imagewise configuration in areas of the member corresponding to the latent image but also is deposited upon non-image areas of the imaging member. However, the magnitude of background attraction in magnetic imaging is typically much less than that in xerographic imaging. For example, typical xerographic background forces acting on xerographic toner are from about 1 to about 2 millidynes which are the magnetic forces exerted in magnetically latently imaged areas. The background forces exerted by magnetically latently imaged members are much less than that of the image areas of the magnetic latent image and the background forces exerted by electrostatically latently imaged members.
These non-image or background areas of the imaging member which bear developing material will transfer these developer materials to the copy medium employed during transfer of the imagewise configured deposition of developing material to the copy medium. Such transfer results in reduced contrast between the transferred image and copy medium and is, therefore, generally undesirable.
Further, in developing latent magnetic images on a magnetizable member it is necessary due to the short range nature of magnetic forces (rapid decrease with distance) to introduce the developing material within a very short distance from the latent magnetic image, typically within about 10 microns of the image, to ensure development of the latent image. This extremely close proximity generally means that developing material will deposit on background areas and, in addition, the latent magnetic image is susceptible to over-development. Over-development, as used herein, means a plurality of layers of magnetic developing material. Over-development of the latent magnetic image is undesirable because, upon transfer to the copy medium, the imagewise configured developing material is susceptible to smearing and smudging, the transferred plurality of layers of developing material is difficult to affix to the copy medium and therefore permanency of the image on the final copy is impaired, and undesired powder clouds of developing material are more likely to be generated within the system when the latent image is over-developed. These powder clouds, in turn, contribute to background and to malfunctioning of imaging apparatus.
The desirability of removing excessive xerographic developing material is well known and recognized in the xerographic art; and is indicated in the magnetic imaging art such as, for example, in U.S. Pat. No. 3,120,806 wherein direct flood developing of the copy medium under the influence of, but out of contact with the latent magnetic image, is followed with air-knife removal of excessive developing material.